1. Field of Invention
The invention relates to a process for determination of the lambda value of reformate which is intended to be supplied to a fuel cell stack, in which the voltage on at least one fuel cell element is evaluated for determination of the lambda value.
Furthermore, the invention relates to a process for lambda control of a reformer for reacting at least fuel and air into reformate which is intended to be supplied to a fuel cell stack.
The invention also relates to a device for determining the lambda value of reformate which is intended to be supplied to a fuel cell stack, the device having means which can evaluate the voltage on at least one fuel cell element for determining the lambda value.
Moreover, the invention relates to a system comprising a reformer for reacting at least fuel and air into reformate and a fuel cell stack which is supplied with reformate by the reformer, the reformer being lambda-controlled.
2. Description of Related Art
The generic processes, devices and systems of the type noted above are used in conjunction with the conversion of chemical energy into electrical energy. For this purpose, fuel and air, preferably in the form of a fuel/air mixture, are supplied to the reformer. The reaction of the fuel with atmospheric oxygen takes place in the reformer, preferably the process of partial oxidation being carried out.
The reformate which has been produced in this way is then supplied to a fuel cell or a fuel cell stack, electrical energy being released by controlled reaction of hydrogen as a component of the reformate, and oxygen.
As already mentioned, the reformer can be designed such that the process of partial oxidation is carried out in order to produce reformate. In this case, when using diesel as fuel, it is especially useful to carry out preliminary reactions before partial oxidation. In this way, long-chain diesel molecules can be reacted into short-chain molecules with a “cold flame;” this ultimately benefits reformer operation. In general, the reaction zone of the reformer is supplied with a gas mixture which is reacted into H2 and CO. Another component of the reformate is N2 from the air, and depending on the air ratio and the temperature, optionally, CO2, H2O and CH4. In normal operation, the fuel mass flow is adjusted according to the required output, and the air mass flow is adjusted to a lambda value or an air ratio in the region of λ=0.4. The reforming reaction can be monitored by different sensors, for example, temperature sensors and gas sensors.
In addition to the process of partial oxidation, it is likewise possible to carry out autothermal reforming. The process of partial oxidation, in contrast to autothermal reforming, is induced by oxygen being substoichiometrically supplied. For example, the mixture has an air ratio of λ=0.4. The partial oxidation is exothermal so that unwanted heating of the reformer can occur in a problematic manner. Furthermore, partial oxidation tends to increased soot formation. To prevent soot formation, the air ratio λ can be chosen to be smaller and/or some of the oxygen used for oxidation made available by water vapor. Since oxidation proceeds endothermally with water vapor, it is possible to adjust the ratio between the fuel, oxygen and water vapor such that altogether heat is neither released nor heat consumed. The autothermal reforming which is achieved in this way therefore eliminates the problems of soot formation and undesirable overheating of the reformer.
It is likewise possible for other steps of gas treatment to take place following oxidation in the reformer, and especially methanation can be downstream of partial oxidation.
One current fuel cell system is, for example, a PEM (proton exchange membrane) system which can typically be operated at operating temperatures between room temperature and roughly 100° C. Due to the low operating temperatures, this fuel cell type is often used for mobile applications, for example, in motor vehicles.
Furthermore, high temperature fuel cells are known, so-called SOFC (“solid oxide fuel cell”) systems. These systems work, for example, in the temperature region of roughly 800° C., a solid electrolyte (solid oxide) being able to take over transport of oxygen ions. The advantage of these high temperature fuel cells compared to PEM systems consists especially in durability relative to mechanical and chemical loads.
One application for fuel cells in conjunction with generic systems includes, besides stationary applications, especially applications in the motor vehicle domain, for example as an auxiliary power unit (APU).
To determine the lambda value of reformate, in the prior art, a sensor (lambda probe) which is provided in the output area of the reformer is often used to measure the oxygen concentration. This constitutes an additional material expenditure which is associated with high costs. Furthermore, tightness problems and/or temperature problems can occur.
3. Description of Related Art
International patent application publication WO 03/094278 A1 discloses generic processes, devices and systems in which a separate lambda probe can be eliminated. According to the teaching of this publication, the lambda value is deduced via the output voltage on one or more fuel cell elements. For this purpose, the pairs of values measured for different operating states from the output voltage and the lambda value are stored. The detection of these pairs of values by measurement engineering is, however, complex and associated with high costs.